In medical X-ray-based imaging, subtraction techniques are used in body regions where there is relatively little movement, e.g., in the field of neurology. This means that the X-ray device first records an anatomical mask image. Then, a slight change is made in the body region, (e.g., a medical instrument visible in X-ray images is moved), a therapeutic and/or diagnostic measure is performed or a contrast medium is injected. If the mask image is then removed from X-ray images that have been recorded with the X-ray device, only the change remains visible. This enables fixed, anatomical structures, (e.g., bones), to be eliminated. It is, for example, possible to generate subtraction images that only show the blood vessel system, a medical instrument, and/or an introduced therapeutic agent, for example, a vessel sealing system in the region of interest of the patient.
Interventions in the blood vessel system, (e.g., in the region of a patient's head), require extremely precise positioning of medical instruments. To this end, methods are known for monitoring the positioning of medical instruments in the imaging, for example, by recording fluoroscopy images as X-ray images of an X-ray device. This enables the movement of medical instruments, which are visible in fluoroscopy images recorded with low X-ray doses, to be tracked. In this context, for navigation applications for interventions in the human body, so-called double-subtraction techniques have been suggested to be used to enhance the display in this context. With a double-subtraction technique, (for example, the so-called “roadmap” technique), first a subtraction technique with contrast media is used to generate a first subtraction image solely depicting the patient's blood vessels in the region of interest. Using this image or a further mask image, a second subtraction technique is used in which an X-ray image showing a medical instrument, for example a guide wire, is recorded and a second subtraction image is generated, which may be superimposed on the first subtraction image to visualize the medical instrument in the blood vessel system.
Like subtraction images, X-ray images are gray-value images, and hence they reproduce the brightness of structures on a gray scale. If two subtraction images are superimposed, (for example, by adding up their gray values), items may be erased in the depiction. For example, a medical instrument shown in a second subtraction image may have a similar gray value to that of the background in an inverted first subtraction image showing the blood vessel system of the patient in the region of interest. Hence, in a subtraction image of this kind inverted before the superimposition, the vessels are not black, but white, so that, for example, on the superimposition of a medical instrument located at the edge of the vessel, clear differentiation from the directly adjacent background may not be possible: information is lost.